Prior art of possible relevance includes the following U.S. Pat. Nos.: 2,932,864 issued Apr. 19, 1960 to Mellen et al; 3,713,475 issued Jan. 30, 1973 to Roelofs et al; and 3,788,380 issued Jan. 29, 1974 to Jasson et al.
Conventional methods of applying a dry particulate material to a precision mold core for forming an investment casting mold include the use of "rainfall" sanders, a fluidized bed, a rotating drum, and manual application. The previously identified Jasson et al and Roelofs et al patents exemplify typical fluidized bed application methods. The rotating drum method is illustrated in the above identified Mellen et al patent. In rainfall sanders, the particulate material is introduced above the mold core and permitted to fall, like raindrops, under the influence of gravity to impinge upon the mold core.
As is well known, the molds are built up in a step by step process on a core which may be formed of a relatively fragile material such as wax. Frequently, ten or more coatings will be required.
Because the density of particulate material is typically much greater in a fluidized bed than in other methods, the application of particulate material can be accomplished more rapidly in such a bed than by other methods. However, in most cases, for the first few coats, fluidized bed application cannot be utilized due to the particulate material scrubbing, and thus deforming, the core. Difficulty may also be encountered in fluidized bed processing of long, narrow mold cores. Such cores are relatively weak and may break in fluidized bed processing if not supported by some buildup of particulate material thereon by prior coatings. When such occurs, the broken part remains in the fluidized bed and, of course, will be subject to the dynamic forces present therein. Because its mass will typically be much greater than that of a typical particle, when the broken part strikes a core, breakage will occur with the consequence that an ever increasing number of broken parts will remain in the bed causing an ever increasing rate of breakage.
These difficulties of fluidized bed processing can be overcome through the use of rainfall sanders or rotary drum applicators. However, application by such means is quite slow and time consuming and therefore more costly.
And irrespective of the type of application employed, various difficulties are attendant their use. In rainfall sanders and rotary drum applicators, the underside of the mold core cannot be coated without rotating or otherwise manipulating and changing the position of the mold core within the unit. In the case of fluidized beds, the upper surface of the core cannot easily be coated without such rotation or manipulation.
Consequently, uneven application of the particulate material may occur or the processor must utilize expensive manipulating equipment to rotate or otherwise shift the position of the core within the processing apparatus. And because such movement of the core is necessary, residence time of a core within the processor must necessarily be increased, thereby slowing down the process and contributing to its expense.